The present invention relates to charge transfer imaging of the type wherein a latent charge image is deposited on a receiving member, and in particular relates to a cartridge for creating the latent charge image. Such cartridges are characterized by having sets of mutually crossing electrodes, that together define a matrix array of charge-generating loci. By way of example, such charge generators may be used to selectively change the state of a planar liquid crystal or other display, or to "write" the latent charge image on a moving dielectric surface, such as a belt or drum, for toning and printing an electrographic image. In this latter area electric charge transfer print cartridges, to compete effectively with other technologies, must exhibit an image resolution of hundreds of dots per inch, or more. This has necessitated the fabrication of electrode arrays having small dimension and very dense packing of elements. For example, a cartridge spanning an 81/2 inch office-size sheet may have over one hundred parallel "finger" electrodes, each having 8, 12 or 16 apertures that define charge transfer loci, within an overall vertical band of about one centimeter width.
The existing print cartridges of this type are versatile, in the sense that the charge generation sites are individually addressable, and their individual outputs may be controlled in magnitude, so that images may be selectively printed or their shading characteristics manipulated or improved by control and image-processing software. Nonetheless, they rely on projecting charged particles across a gap to the receiving member, and the quantity of charge received at the member thus critically depends on maintaining a proper gap and uniform alignment over the active area of the cartridge.
Since the introduction of the earliest cartridges of this type, as exemplified, for example, in U.S. Pat. Nos. 4,155,093 to Fotland and Carrish and 4,160,257 to Carrish, these cartridges have been fabricated with a rigidifying member to provide the necessary stiffness and dimensional stability.
In a typical construction the cartridge is located adjacent a dielectric surface of a drum, oriented parallel to the drum axis, at a spacing of 0.2 to 0.5 mm. from the surface. When a belt is used rather than an imaging drum, the belt typically passes over a drum or over a flat platen, which holds it in a precise physical location opposed to the cartridge and which generally also defines a conductive backplane that establishes an accelerating potential to move the charge carriers from the cartridge to the imaging member.
These cartridges operate by controlled generation of localized plasma discharges, and in use are subject to heating up; they must also be mounted so that their many electrical contacts are dependably maintained without introducing mechanical stresses that might deform the cartridge along its span. If dimensional changes in the cartridge to imaging member gap do occur, they can result in flashover or arcing when the gap decreases, or loss of intensity or resolution when the gap increases.
One example of cartridge construction is described in commonly owned U.S. Pat. No. 4,679,060 to McCallum et al. This cartridge includes a number of relatively thin planar structural layers and produces a charge transfer image by means of a charge generator in the form of a matrix of electrodes located on an inner surface of the cartridge. Outer surfaces of the cartridge facing away from the drum are provided with contacts for electrical connection of individual electrodes with corresponding spring biased contacts linked to a cartridge control board, also known as a mother board, for controlling image generation. An exemplary configuration of a drum-type printer for receiving such a cartridge is described in U.S. Pat. No. 4,516,847 to Maczuszenko et al. That cartridge also includes an aluminum spine which rigidifies the cartridge and extends outwardly to provide a handle to be used when the cartridge is being fitted or removed from the printer.
That cartridge is mounted in a printer on mounting blocks which are adjusted relative to rigid parts of the printer structure using shims to give the desired spacing between the cartridge and the drum surface (typically 0.01 inches). Understandably, it would be difficult to adjust the spacing each time a cartridge was replaced. Accordingly, the mounting blocks are set-up during assembly of the printer and are not normally adjusted during the life of the printer, so that replacement cartridges must be accurately located on the mounting blocks. To achieve this accurate location, the lower contact surface of each cartridge must be accurately sized and is, therefore, formed of a substantial piece of high grade material, typically high grade fibre glass reinforced epoxy, which adds considerably to the cost of the cartridge. Also, particles of dust or the like may find their way between the contact surfaces of the cartridge and mounting and thus affect the spacing.
Connections between the contacts on the outer face of the cartridge and the mother board are made by spring pin contacts which extend downwardly from the mother board. These contacts are relatively expensive and the total cost of the hundreds of contacts required for a cartridge adds significantly to the total cost of the printer. Also, the spring forces exerted on the cartridge contacts by the spring contacts further complicate the accurate location of the cartridge because the accumulation of small forces tends to push the cartridge towards the drum, and could affect the spacing between the cartridge and drum.
Other forms of cartridges are available which provide cartridge contacts on the inner face of the cartridge and do not require such expensive spring pin contacts. However, the mother board contacts for such cartridges must be located in the restricted space between the cartridge and the drum, the space becoming more restricted as larger diameter print drums are utilized. These cartridges also suffer from the disadvantage that the spring forces from the mother board contacts tend to push the cartridge away from the drum, and again could affect the spacing between the cartridge and the drum.
These disadvantages have lead to the design of a different configuration of cartridge and cartridge mounting, as described, for example, in commonly-owned U.S. patent entitled Charge Transfer Imaging Cartridge Mounting and Printer, U.S. Pat. No. 4,951,070. In one such mounting, the cartridge includes a rectangular cross section spine, the inner portion of the cartridge being located on a face of the spine and the cartridge contacts being located on side faces of the spine. The cartridge is located in a channel defined by two spaced elements, from which spring biased mother board contacts extend to bear against the cartridge contacts. As the spring forces from the mother board contacts are acting on the cartridge parallel to the inner face and the drum surface, the forces do not tend to affect the spacing between the cartridge and the drum.
In order to preserve the benefits of planar fabrication of these devices, and yet not exert forces perpendicular to the electrode plane for contacting the various electrodes, a further cartridge exemplified in U.S. Pat. No. 5,030,975 was developed. That cartridge was formed by a planar electrode matrix fabricated on a flexible dielectric substrate. With this arrangement, the completed electrode matrix could be bent around a rigid box spine, and its electrodes easily accessed and contacted on the sides or back of the spine.
This latter construction has achieved a very dense dot array with a high degree of surface flatness in a cartridge that is not prone to warpage in use.
However, while the array remains flat, the processing and wrapping of the flexible array about the rigid spine has now been found to introduce stresses into the structure that could affect alignment of the electrodes defining individual dot sites. The effect of such stresses introduced during manufacturing is an irregular shifting of the electrode layers of the device such that the active electrodes or corona recesses become misaligned, leading to loss of output.